The fast uptake of LTE in different regions of the world shows that the demand for wireless broadband data is increasing, and that the LTE is an extremely successful platform to meet that demand. At the same time, unlicensed spectrum is more and more considered by cellular operators as a complementary tool to augment their service offering. Therefore, in 3GPP TSG RAN #65 meeting, the Licensed-Assisted Access using LTE (LAA-LTE) was approved for Release 13 study [1].
For LAA-LTE, a critical element of the design is to ensure LAA co-existence in unlicensed band with other operators and other access technologies such as Wi-Fi on “fair” and “friendly” bases. Different regions have different regulatory requirements for unlicensed band operation as summarized in [2]. For example, in Europe, the regulations mandate the equipment operating on unlicensed spectrum to implement Listen-Before-Talk (LBT) by performing Clear Channel Assessment (CCA) before starting a transmission, so as to verify that the operating channel is not occupied. Further, two types of LBT behaviors are defined by ETSI rules: Frame-Based Equipment (FBE) and Load-Based Equipment (LBE). For FBE, CCA check is performed once every fixed frame period. If the channel is not available, the transmitter will have to wait for the entire period before the next channel access opportunity. For LBE, the eNB performs CCA check once to obtain channel access. If the channel is not available, the eNB performs an Extended CCA (eCCA). Once the eNB obtains the channel and performs a transmission, the eNB has to perform an extended CCA to access the channel again. The main differences of allowed Channel Occupancy Time and Idle Time are listed in Tab.1. As each eNB using LBE can sense the channel at any time, LBE based access may be fairer for eNBs of different operators in comparison with FBE based on access [3].
TABLE 1Comparison between FBE and LBEChannelOccupancy TimeIdle timeFBEBetween 1 ms and 10 msAt least 5% of Occupancy time(500 us for 10 ms Occupancy time)LBE<=(13/32) × q ms,At least 20 us CCA observationq = 4 . . . 32time if the operating channel is freebefore transmission, otherwise includingextra eCCA observation time
However, for LBE, the LAA-LTE transmitter need perform a CCA or an Extended CCA to determine if the channel is available. Due to the random nature of interference experienced in the unlicensed spectrum, the LAA-LTE transmitter (eNB for DL and user equipment (UE) for UL) may acquire the medium at any time possibly in the middle of a subframe, and thus the end of the CCA or eCCA is not aligned with an OFDM symbol or a subframe boundary. Unlike Wi-Fi system, LTE system is a scheduling system with strict frame structure. To comply with LBT, the frame structure modifications are required for LAA-LTE so as to allow flexible channel sensing and occupancy in unlicensed bands for LBE, while still keeping or even enhancing advantages of the LTE system.